Staging: at76_usb: convert to use linux/ieee80211.h

Jason Andryuk noticed that at76_usb won't compile with current
wireless-testing tree. This is because net/ieee80211.h was missing.

net/ieee80211.h will be removed soon and at76_usb won't compile then that
happens. Preprare for that by using instead linux/ieee80211.h and copying
some structures not available.

Signed-off-by: Kalle Valo <kalle.valo@iki.fi>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>